Guide to writing output filters

There are a number of common pitfalls encountered when writing
output filters; this page aims to document best practice for
authors of new or existing filters.

This document is applicable to both version 2.0 and version 2.2
of the Apache HTTP Server; it specifically targets
RESOURCE-level or CONTENT_SET-level
filters though some advice is generic to all types of filter.

Each time a filter is invoked, it is passed a bucket
brigade, containing a sequence of buckets which
represent both data content and metadata. Every bucket has a
bucket type; a number of bucket types are defined and
used by the httpd core modules (and the
apr-util library which provides the bucket brigade
interface), but modules are free to define their own types.

Output filters must be prepared to process
buckets of non-standard types; with a few exceptions, a filter
need not care about the types of buckets being filtered.

A filter can tell whether a bucket represents either data or
metadata using the APR_BUCKET_IS_METADATA macro.
Generally, all metadata buckets should be passed down the filter
chain by an output filter. Filters may transform, delete, and
insert data buckets as appropriate.

There are two metadata bucket types which all filters must pay
attention to: the EOS bucket type, and the
FLUSH bucket type. An EOS bucket
indicates that the end of the response has been reached and no
further buckets need be processed. A FLUSH bucket
indicates that the filter should flush any buffered buckets (if
applicable) down the filter chain immediately.

FLUSH buckets are sent when the
content generator (or an upstream filter) knows that there may be
a delay before more content can be sent. By passing
FLUSH buckets down the filter chain immediately,
filters ensure that the client is not kept waiting for pending
data longer than necessary.

Filters can create FLUSH buckets and pass these
down the filter chain if desired. Generating FLUSH
buckets unnecessarily, or too frequently, can harm network
utilisation since it may force large numbers of small packets to
be sent, rather than a small number of larger packets. The
section on Non-blocking bucket reads
covers a case where filters are encouraged to generate
FLUSH buckets.

Example bucket brigade

HEAP FLUSH FILE EOS

This shows a bucket brigade which may be passed to a filter; it
contains two metadata buckets (FLUSH and
EOS), and two data buckets (HEAP and
FILE).

For any given request, an output filter might be invoked only
once and be given a single brigade representing the entire response.
It is also possible that the number of times a filter is invoked
for a single response is proportional to the size of the content
being filtered, with the filter being passed a brigade containing
a single bucket each time. Filters must operate correctly in
either case.

An output filter which allocates long-lived
memory every time it is invoked may consume memory proportional to
response size. Output filters which need to allocate memory
should do so once per response; see Maintaining
state below.

An output filter can distinguish the final invocation for a
given response by the presence of an EOS bucket in
the brigade. Any buckets in the brigade after an EOS should be
ignored.

An output filter should never pass an empty brigade down the
filter chain. To be defensive, filters should be prepared to
accept an empty brigade, and should return success without passing
this brigade on down the filter chain. The handling of an empty
brigade should have no side effects (such as changing any state
private to the filter).

How to handle an empty brigade

A bucket brigade is a doubly-linked list of buckets. The list
is terminated (at both ends) by a sentinel which can be
distinguished from a normal bucket by comparing it with the
pointer returned by APR_BRIGADE_SENTINEL. The list
sentinel is in fact not a valid bucket structure; any attempt to
call normal bucket functions (such as
apr_bucket_read) on the sentinel will have undefined
behaviour (i.e. will crash the process).

There are a variety of functions and macros for traversing and
manipulating bucket brigades; see the apr_buckets.h
header for complete coverage. Commonly used macros include:

APR_BRIGADE_FIRST(bb)

returns the first bucket in brigade bb

APR_BRIGADE_LAST(bb)

returns the last bucket in brigade bb

APR_BUCKET_NEXT(e)

gives the next bucket after bucket e

APR_BUCKET_PREV(e)

gives the bucket before bucket e

The apr_bucket_brigade structure itself is
allocated out of a pool, so if a filter creates a new brigade, it
must ensure that memory use is correctly bounded. A filter which
allocates a new brigade out of the request pool
(r->pool) on every invocation, for example, will fall
foul of the warning above concerning
memory use. Such a filter should instead create a brigade on the
first invocation per request, and store that brigade in its state structure.

It is generally never advisable to use
apr_brigade_destroy to "destroy" a brigade unless
you know for certain that the brigade will never be used
again, even then, it should be used rarely. The
memory used by the brigade structure will not be released by
calling this function (since it comes from a pool), but the
associated pool cleanup is unregistered. Using
apr_brigade_destroy can in fact cause memory leaks;
if a "destroyed" brigade contains buckets when its
containing pool is destroyed, those buckets will not be
immediately destroyed.

In general, filters should use apr_brigade_cleanup
in preference to apr_brigade_destroy.

When dealing with non-metadata buckets, it is important to
understand that the "apr_bucket *" object is an
abstract representation of data:

The amount of data represented by the bucket may or may not
have a determinate length; for a bucket which represents data of
indeterminate length, the ->length field is set to
the value (apr_size_t)-1. For example, buckets of
the PIPE bucket type have an indeterminate length;
they represent the output from a pipe.

The data represented by a bucket may or may not be mapped
into memory. The FILE bucket type, for example,
represents data stored in a file on disk.

Filters read the data from a bucket using the
apr_bucket_read function. When this function is
invoked, the bucket may morph into a different bucket
type, and may also insert a new bucket into the bucket brigade.
This must happen for buckets which represent data not mapped into
memory.

To give an example; consider a bucket brigade containing a
single FILE bucket representing an entire file, 24
kilobytes in size:

FILE(0K-24K)

When this bucket is read, it will read a block of data from the
file, morph into a HEAP bucket to represent that
data, and return the data to the caller. It also inserts a new
FILE bucket representing the remainder of the file;
after the apr_bucket_read call, the brigade looks
like:

The basic function of any output filter will be to iterate
through the passed-in brigade and transform (or simply examine)
the content in some manner. The implementation of the iteration
loop is critical to producing a well-behaved output filter.

The above implementation would consume memory proportional to
content size. If passed a FILE bucket, for example,
the entire file contents would be read into memory as each
apr_bucket_read call morphed a FILE
bucket into a HEAP bucket.

In contrast, the implementation below will consume a fixed
amount of memory to filter any brigade; a temporary brigade is
needed and must be allocated only once per response, see the Maintaining state section.

A filter which needs to maintain state over multiple
invocations per response can use the ->ctx field of
its ap_filter_t structure. It is typical to store a
temporary brigade in such a structure, to avoid having to allocate
a new brigade per invocation as described in the Brigade structure section.

If a filter decides to store buckets beyond the duration of a
single filter function invocation (for example storing them in its
->ctx state structure), those buckets must be set
aside. This is necessary because some bucket types provide
buckets which represent temporary resources (such as stack memory)
which will fall out of scope as soon as the filter chain completes
processing the brigade.

To setaside a bucket, the apr_bucket_setaside
function can be called. Not all bucket types can be setaside, but
if successful, the bucket will have morphed to ensure it has a
lifetime at least as long as the pool given as an argument to the
apr_bucket_setaside function.

Alternatively, the ap_save_brigade function can be
used, which will move all the buckets into a separate brigade
containing buckets with a lifetime as long as the given pool
argument. This function must be used with care, taking into
account the following points:

On return, ap_save_brigade guarantees that all
the buckets in the returned brigade will represent data mapped
into memory. If given an input brigade containing, for example,
a PIPE bucket, ap_save_brigade will
consume an arbitrary amount of memory to store the entire output
of the pipe.

When ap_save_brigade reads from buckets which
cannot be setaside, it will always perform blocking reads,
removing the opportunity to use Non-blocking
bucket reads.

If ap_save_brigade is used without passing a
non-NULL "saveto" (destination) brigade parameter,
the function will create a new brigade, which may cause memory
use to be proportional to content size as described in the Brigade structure section.

Filters must ensure that any buffered data is
processed and passed down the filter chain during the last
invocation for a given response (a brigade containing an EOS
bucket). Otherwise such data will be lost.

The apr_bucket_read function takes an
apr_read_type_e argument which determines whether a
blocking or non-blocking read will be performed
from the data source. A good filter will first attempt to read
from every data bucket using a non-blocking read; if that fails
with APR_EAGAIN, then send a FLUSH
bucket down the filter chain, and retry using a blocking read.

This mode of operation ensures that any filters further down the
filter chain will flush any buffered buckets if a slow content
source is being used.

A CGI script is an example of a slow content source which is
implemented as a bucket type. mod_cgi will send
PIPE buckets which represent the output from a CGI
script; reading from such a bucket will block when waiting for the
CGI script to produce more output.

Output filters should not pass empty brigades down the filter
chain, but should be tolerant of being passed empty
brigades.

Output filters must pass all metadata buckets down the filter
chain; FLUSH buckets should be respected by passing
any pending or buffered buckets down the filter chain.

Output filters should ignore any buckets following an
EOS bucket.

Output filters must process a fixed amount of data at a
time, to ensure that memory consumption is not proportional to
the size of the content being filtered.

Output filters should be agnostic with respect to bucket
types, and must be able to process buckets of unfamiliar
type.

After calling ap_pass_brigade to pass a brigade
down the filter chain, output filters should call
apr_brigade_cleanup to ensure the brigade is empty
before reusing that brigade structure; output filters should
never use apr_brigade_destroy to "destroy"
brigades.

Output filters must setaside any buckets which are
preserved beyond the duration of the filter function.

Output filters must not ignore the return value of
ap_pass_brigade, and must return appropriate errors
back up the filter chain.

Output filters must only create a fixed number of bucket
brigades for each response, rather than one per invocation.

Output filters should first attempt non-blocking reads from
each data bucket, and send a FLUSH bucket down the
filter chain if the read blocks, before retrying with a blocking
read.

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